One of the factors involved in the manufacture of semiconductor devices is a depth of focus (DOF) window. Generally, an effective DOF will cover all the variations of photoresist thickness, local substrate topology step height, and wafer center and edge step height differences. An effective DOF facilitates manufacturing a semiconductor device within a desired critical dimension (CD) specification with little or no scum or top loss defects.
Problems may occur with photoresist that is thicker than the DOF. For example, if the DOF is less than the thickness of the photoresist layer plus step height variation, scum or CD errors may occur in some of the patterns formed on the semiconductor devices. Therefore, thin layers of photoresist may be desired to counter this problem. Such thin photoresist layers may also be desirable for low dosage exposure tools, such as an e-beam or extreme ultraviolet (EUV) tools, as they may improve resist contrast, resolution, and dissolution. Moreover, for mass production purposes, the combination of thin photoresist layers and low dosage exposure tools can increase the throughput of semiconductor devices.
However, the use of thin photoresist layers can have drawbacks. For example, a thin photoresist layer may adversely affect etching performance if it does not provide sufficient protection during the etch process. To resolve this problem, a two step process may be used. For example, a relatively thin photo sensitive layer may be formed over a thick buffer layer. The photo sensitive layer is developed to form a predefined pattern, and the buffer layer is then etched to correspond to the pattern formed by the photo sensitive layer. The buffer layer then serves as an etch stop layer during etching of the substrate. Accordingly, two removal steps (developing and etching) are needed to reach the substrate prior to etching the substrate.
Therefore, what is needed is a new and improved photolithography process to address these drawbacks.